1-amino-1(1H-tetrazol-5-yl)-azo-guanidine hydrate (“Tetrazene,” 1, Scheme 1) is widely used in ordnance systems as a sensitizer of primer mixes for use in both percussion and stab applications.
Scheme 1:
It has low thermal and hydrolytic stability compared with other components of primer mixes and there currently exists the need for a replacement with enhanced stability characteristics.
This material was initially prepared in 1910 by Hoffman and Roth, Ber. Dtsch. Chem. Ges., 43, 682 (1910), and the molecular structure was determined in 1971. Duke, J. R. C., “X-Ray Crystal and Molecular Structure of Tetrazene”, J. Chem. Soc. D Chemical Communications, 2 (1971). Tetrazene was first used in primer compositions in Germany in the 1920s due primarily to the work of Von Herz who demonstrated it to be non-corrosive as well as free of the toxic mercury fulminate used up to that time. In the United States, mercury fulminate-based primers were initially replaced with FA70, which contained potassium chlorate, lead thiocyanate and TNT. Fedoroff, Encyclopedia of Explosives and Related Items, Vol. 8, P373 (S. M. Kaye ed., US Army ARDEC 1978). These compositions, although useful for high temperature applications, also proved corrosive and were replaced in 1948 by FA956, which was developed by Rathsburg and contained Tetrazene and lead styphnate.
Current primer mixes, such as NOL-130, are generally comprised of basic lead styphnate 40%, lead azide 20%, barium nitrate 20%, antimony sulfide 15%, and Tetrazene 5%. Cooper, P. W., Explosives Engineering, 323-326 (Wiley-VCH, New York, 1996). However, mixes containing other components are known. Id. (teaching use of VH2 and L Mix); Federoff, supra (teaching use of PA100).
The NOL-130 composition is relatively insensitive to stab initiation when the Tetrazene component is omitted. The addition of ≧2% Tetrazene is currently a requirement to ensure high stab sensitivity. It has been proposed that the low mechanical energy requirements for initiation of Tetrazene-based compositions is a direct result of its low temperature of ignition (143° C.) in conjunction with its metastable chemical structure. Bird, R., “The Stab Sensitizing Action of Tetrazene,” Materials Research Laboratories Technical Note, 362 (1975). This may be related to the mechanism of initiation where mechanical energy is applied to the primer and is converted to heat by friction and impact events between the explosive materials and/or grits in the composition. Field, J., “Hot Spot Ignition Mechanisms for Explosives,” Acc. Chem. Res., 25, 489 (1992). Spear and Elischer have investigated 17 compounds as alternates to Tetrazene for sensitizing lead azide. Spear, R. J. and Elischer, P.P., “Studies on Stab Initiation. Sensitization of Lead Azide by Energetic Sensitizers”, Aust. I Chem., 35, 1 (1982). They correlated ignition temperature with initiation energies.
Tetrazene suffers from a number of issues, most importantly low thermal and hydrolytic stabilities. It has been demonstrated that Tetrazene readily decomposes at approximately 90° C., which is in the temperature range that may be encountered during storage and handling in some parts of the world. As illustrated in Scheme 2 below, Bird has demonstrated using IR and UV monitoring that at 90° C., 1 mol of Tetrazene decomposes to afford 1.7 mol of 5-aminotetrazole (2, Scheme 2) via a fragmentation process followed by cyclization of a majority of the guanyl azide intermediate. Bird, R. and Power, A. J., “Thermal Decomposition of Tetrazene at 90° C.,” Materials Research Laboratories Report MRL-R-710 (1978).
Scheme 2:

This decomposition is complete after 1 week at 90° C. and appears to be auto-catalytic. Thus, it is reasonable to presume that it will occur to some extent at lower temperatures. More recent investigations have confirmed these results. Whelan, D. J. and Fitzgerald, M. R., “The Kinetics and Thermochemistry of the Thermal Decomposition of the Initiating Explosive, Tetrazene,” DSTO Aeronautical and Maritime Research Laboratory Report DSTO-TR-0450 (1996). Tetrazene has also been shown to be susceptible to hydrolytic decomposition and is destroyed/decomposed simply by addition to boiling water. Spear and Elischer, supra.
As a result, it is clear that there is a need to develop a stab sensitive material with improved thermal stability for primers used in normal or high temperature applications. In light of current environmental concerns, it may also be desirable to generate this new material both inexpensively and without the use of toxic materials as reactants or effluents.